In networks, tandem connection monitoring can provide an administrative monitoring domain that operates independent of the end to end path. Tandem Connection Monitoring (TCM) is used in Synchronous Optical Network/Synchronous Digital Hierarchy (SONET/SDH) networks to support path monitoring between various tandem connected network elements. In accordance with existing ITU recommendations regarding SDH networks, TCM allows monitoring of the performance of path segments with the aid of the network operator bytes N1, N2 in the path over head (POH). The high-order and low-order POH parity bytes are evaluated by the network elements. The number of errors detected is indicated to the end of the TCM trail using the N1 or N2 byte. This error count is then recompared with the number of parity errors detected at the end of the TCM trail. The difference is the number of errors occurring within the TCM trail.
With the ever increasing use of hierarchal networks including many smaller networks that may be owned by different network operators, multi-level TCM has gained popularity as a means of improving network monitoring and operator accountability. The ITU has proposed requirements (ITU final draft recommendation ITU-T G.709) for Optical Transport Module (OTM) signals of an Optical Transport Network (OTN) that defines an Optical Channel Data Unit (ODU) frame structure having an overhead that includes portions dedicated to the end-to-end ODU path and six levels of tandem connection monitoring. The ODU path overhead is terminated where the ODU is assembled and disassembled. The TCM overhead is added and terminated at the source and sink of the corresponding tandem connections, respectively. In this regard, the ODUk path overhead includes six TCM fields such that the number of monitored connections along an ODU trail may vary between 0 and 6 and the monitored connections can be nested, overlapping, and/or cascaded.
In some instances, multiple trail terminations will be provided at a single network element. One proposed method of multi-level TCM processing at a single network element is to process the TCM data serially through the trail terminations existing at the network element. This can result in incorrect TCM results when terminating TCM trails overlap within the network element. For example, a first TCM trail may commence at a source located upstream of a mis-connection, and terminate at a sink in a network element located downstream of the mis-connection. A second TCM trail could commence and terminate downstream of the mis-connection, with the sink of the second TCM trail being at the same network element, but downstream of, the sink of the first TCM trail. Although no mis-connection actually existed in the second TCM trail, the sink of the first TCM trail would properly detect the mis-connection, and would generate a downstream Alarm Indication Signal (AIS). This AIS will be detected by the sink of the second TCM trail, causing it to generate a signal failure alarm for the second TCM trail even though the second TCM trail has not been directly affected by the mis-connection. Accordingly, in a system where TCM trail terminations are serially processed in a network element, the order in which TCM trails are added or dropped within the network element is critical as some configurations can result in unnecessary alarms. This reduces flexibility in configuring the transport system.
Thus, it is desirable to provide an improved Tandem Connection Monitoring system and method which is unaffected by the order in which trail termination points are added and dropped within a network element.